1. Field of the Invention
The invention relates to a light source which is used in a photolithography process or the like in the production of semiconductors, liquid crystal cells and the like, or a light source for a projection apparatus using “digital micro mirror devices” and liquid crystals and the like, which is used for image projection system. The invention relates especially to the electrodes of a short arc discharge lamp.
2. Description of the Prior Art
Conventionally, for the cathode of a short arc discharge lamp, a metallic material with a high melting point is used and it is doped with an emissive material, for example, thoriated tungsten, i.e, tungsten which is doped with thorium oxide. This cathode is formed by the tip area of a cylindrical material being processed to be conical. The outermost tip area of the cathode is formed flat or spheroidal, so that a current density suitable to the lamp current is obtained. In this short arc discharge lamp, in the vicinity of the cathode tip, the highest radiance is achieved during luminous operation. The reason for this is that the vicinity of the cathode tip has the smallest diameter of the diameters of this cathode and that the highest current density is achieved here.
An optical device in which such a short arc discharge lamp is installed is conventionally built such that the focal point of the optical device agrees with the arc radiance spot part which is located in the vicinity of the cathode tip so that the area with high radiance of the cathode tip can be used with high efficiency. Generally, when the short arc discharge lamp is installed, the lamp position is set such that the maximum illuminance of the optical system is achieved. However, in the above described short arc discharge lamp, it is regarded as a disadvantage that the cathode is worn off over the period of illumination and the position of the arc radiance spot part on the tip of the cathode is shifted backwards in the direction toward an end of the lamp, for example, in the case of luminous operation, in an arrangement of the cathode in the lower area of the lamp, over the period of illumination, to the bottom. Therefore, the optimum focal point of the optical device is shifted and the illuminance of the light emitted by the optical system deteriorates dramatically. The reason for this is, presumably, that the current density is high on the cathode tip and the cathode temperature increases greatly. Conventionally, countermeasures are taken against this disadvantage by adjusting the lamp installation site of the optical device more often. But, frequent adjustment is not only complex, but during this adjustment, further adjustments must be made each time in addition to adjusting the lamp position, for example, adjusting of the exposure time or the like. As a result, the number of working steps is greatly increased.
As a process for preventing wear of the cathode tip, it can be imagined that the diameter of the cathode can be increased and the current density of the cathode tip decreased. However, if the current density is reduced, there is the disadvantage that the radiance of the arc radiance spot decreases. Furthermore, there are the disadvantages that an arc discharge takes place in which the discharge is locally polarized on the cathode tip and the arc discharge moves vigorously on the cathode tip and therefore becomes unstable. This instability of the arc discharge has the disadvantage that the emitted light becomes very unstable; for example, this causes nonuniform illuminance on an exposure surface in an exposure apparatus or the like, and in a projection apparatus or the like, leads to flickering of the projection images.
Furthermore, there is the disadvantage that, a tungsten rod is used as the material of the cathode in the above described short arc discharge lamp, mainly a thoriated tungsten rod, in which 2% by mass thorium oxide is normally added, and this thorium oxide is an emissive material and easily emits electrons during discharge. Thus, if the thorium oxide is not uniformly supplied over the entire cathode tip, both in terms of time and space, broadening and also contraction of the arc radiance spot, furthermore local concentration of the discharge and the like, occur. The thorium oxide is supplied either by diffusion along the grain boundaries of the tungsten material or by surface diffusion of the thorium oxide which has been deposited on the cathode surface. With respect to supply by surface diffusion, for example, Japanese Patent JP 2782610 B2 discloses that part of the surface of the conical area of the cathode tip is subjected to carbonization treatment. But, this process is also used to increase the amount of thorium oxide deposited on the side of the cathode. In this case, the above described supply is ensured by surface diffusion. The above described supply along the grain boundaries is however not ensured. Moreover, there is the disadvantage that a reduction in the supply amount of thorium oxide causes broadening of the arc radiance spot in the cathode tip area and that, conversely, for an excess supply amount of the thorium oxide, contraction of the arc radiance spot in the cathode tip area and flickering of the arc discharge occur.
FIGS. 5(a) and 5(b) each show an arc discharge state at the start of luminous operation and the crystal state of the cathode tip in a conventional short arc discharge lamp. FIG. 5(a) shows a discharge state on a conventional cathode with a relatively thin tip shape and the crystal state of this cathode. The particle size—hereinafter also called the crystal grain size—in the cathode tip area is roughly as large as on the side of the lead pin and is relatively small. For this relatively small crystal grain size, supply by diffusion of the thorium oxide along the grain boundaries is effective, as was described above. At the start of luminous operation there is a state of excess supply. The arc discharge encompassed the entire tip area, but the stability of the arc discharge was low. Since the diameter of the cathode tip is small and the current density on this cathode tip is high, and for similar reasons, the cathode tip area was worn to a great extent. This pushed the position of the cathode tip backwards, and in the optical system, an extreme reduction of the illuminance occurred.
FIG. 5(b) shows a case in which the tip was made thicker than in the case of FIG. 5(a), and in which the attempt was made to reduce the current density acting on this cathode. The particle size in the cathode tip area at the start of luminous operation is, as in FIG. 5(a), roughly as large as on the side of the lead pin and is relatively small. Stability of the arc discharge is lacking as in FIG. 5(a). The arc discharge shown in the drawings relates to a case in which it is polarized in part of the cathode tip. When this arc discharge is locally concentrated and manifested, the temperature of the cathode tip is rather nonuniform; this promotes wear of the cathode. Furthermore, if the polarized arc discharge is not manifested at one location, the emitted light becomes very unstable; in an exposure apparatus and the like, this causes the disadvantage of nonuniformity of the illuminance on the exposure surface and in a projection apparatus and the like, the disadvantage of flickering of the projection images.